This invention relates to a fastening element for use with a tie member to fasten a first component to a second component, to an indicator element for use with such a fastening element to indicate fastness of said components, and to a method of securing a first component to a second component using such an element and a tie member.
It is well known to fasten two or more components together by passing a bolt through the components and tightening a nut onto the bolt, so that the bolt head and nut each exert a clamping force on the components.
Vibration and flexure of the components tend to loosen the nut on the bolt, and such loosening is prevented only by friction between the bolt head and nut and the components.
Where the components are relatively compliant, the nut may have to be loosened by up to three turns before all of the clamping force on the components is lost. Where, on the other hand, the components are relatively stiff, the nut may have to be loosened by as little as one ninth of a turn before all of the clamping force is lost. The loss of the clamping force substantially reduces the friction between the bolt head and nut and the components, allowing the nut to move freely relative to the bolt until the components eventually separate.
It is known to interpose a split or spring washer between the nut and the component, the washer being compressed by the clamping force exerted on the components and expanding against the component and nut in the event of a momentary reduction of the clamping force, for example due to flexure of the components, so as to prevent the nut from loosening. However, the use of split or spring washers is not particularly effective and introduces an undesirable complication into an assembly line.
A particular combination of components, nut and bolt will have an optimum clamping force, which gives a high degree of friction between the bolt head, nut and components, but avoids excessive distortion of the components, snapping the bolt or stripping the threads of the bolt or nut.
Once the optimum clamping force has been determined, taking into account thread pitch, bolt diameter, loaded bolt length, and lubricity, surface finish and dimensional variation of the components, the clamping force is obtained by applying a predetermined torque to the nut.
At present, the only method of determining whether a nut has been correctly tightened is by means of a torque spanner. However, if the nut has been correctly tightened, the application of the torque spanner to the nut will cause the nut to be further tightened, and the optimum clamping force to be exceeded.
According to a first aspect of the invention there is provided a fastening element for use with a tie member to fasten a first component to a second component, the fastening element being adapted for attachment to the tie member, which in use passes through a part of at least one of the components, and for cooperating with the tie member to exert a compressive force on the components, characterised in that the element comprises an inner portion resiliently joined to an outer portion, in such a way that, when a tensile reaction force is applied to the tie member as a result of said compressive force, said force causes the outer portion to be displaced relative to the inner portion.
The invention therefore provides a fastening element for use with a tie member, which element enables the magnitude of a tensile force applied to the tie member to be determined without altering the magnitude of the force, and which is operable to prevent relative movement of the fastening element and the tie member in the event of a momentary reduction of the compressive force on the components.
Preferably the inner portion is formed integrally with the outer portion.
Preferably the relative position of the inner and outer portions is such that the displacement of the outer portion relative to the inner portion is an axial displacement.
The fastening element may be attached to the tie member by any convenient means, for example crimping onto, or welding to, the member, or may be integrally formed with the tie member. Preferably, however, the inner portion is tapped so as to be engageable with a threaded portion at a first end of the member.
Said compressive force may be exerted by means of a nut tightened against the second component onto a threaded portion at a second end of the member, but preferably the outer portion of the element is formed with flats for engagement with a spanner or like tool, such that the compressive force and hence the tensile reaction force may be applied by means of tightening the element, against the first component, onto the threaded portion at the first end of the member. In this case the second end of the member may be formed with a head for engagement with the second component, or be fastened to the second component, for example by welding, or the member may be integrally formed with the second component.
The fastening element may advantageously be formed from sheet or strip spring steel.
Preferably, the fastening element comprises a nut, for use with a tie member in the form of a bolt or stud.
The fastening element may advantageously further comprise a washer that is rotatably attached to the nut, and that, in use, is interposed between the nut and the components.
Preferably the washer is annular, such that in use, the tie member passes through the washer.
The fastening element may advantageously further comprise cage means having first and second pluralities of axial, radially inwardly projecting claws, the first plurality of claws being engageable with the nut and the second plurality of claws being engageable with the washer.
In a preferred embodiment of the fastening element the nut is formed with a generally frusto-conical portion and the annular washer is formed with a continuous groove in its external circumference, and the first plurality of claws of the cage are engageable with the frusto-conical portion of the nut and the second plurality of claws are engageable with the groove of the washer.
Preferably the washer is formed from hardened steel and the cage is formed from mild or spring steel.
In this way during tightening of a tapped fastening element onto a threaded tie member against a component, the nut rotates against the washer rather than against the component, and there is little, if any, relative movement between the washer and the component, such that scoring of the surface of the component is much reduced.
According to a second aspect of the invention there is provided an indicator element for use with a fastening element in accordance with the first aspect of the invention, the indicator element being releasably engageable with a fastening element such that the indicator element is retained by the fastening element when the outer portion of the fastening element is displaced relative to the inner portion, and released if the displacement of the outer portion relative to the inner portion decreases substantially.
The second aspect of the invention therefore provides an indicator element that may be engaged with a fastening element in accordance with the first aspect of the invention, once the fastening element is exerting a desired compressive force on a component, but which will be released from the fastening element if the compressive force decreases by a substantial amount, for example due to loosening of the fastening element, and hence provides a visual indication of the decrease in the compressive force.
Moreover, the indicator element enables a very quick check to be carried out of whether a fastening element is exerting a desired compressive force on a component, simply by attempting to engage the indicator element with the fastening element, since the indicator element will be retained only if the fastening element is exerting the desired compressive force.
The indicator element may advantageously be adapted to be engageable with an internal surface of the outer portion of the fastening element when the outer portion is displaced relative to the inner portion.
In a preferred embodiment the indicator element comprises a cap for placing over the fastening element, the cap having a central internal tubular member with a perpendicularly outwardly projecting lip that is engageable with an internal surface of the outer portion of the fastening element when the outer portion is displaced relative to the inner portion. When the outer portion is not displaced relative to the inner portion, the external surface of the inner portion occludes the internal surface of the outer portion, such that the lip of the tubular member cannot engage with the internal surface of the outer portion.
The indicator element may advantageously be formed from a plastics material.
Preferably the indicator element is formed from nylon-66.
According to a third aspect of the invention there is provided a method of securing a first component to a second component, by means of a fastener comprising an element as hereinabove described and a tie member, the method comprising the steps of applying a progressively increasing compressive force to the two components, by means of the fastener, and determining whether said displacement of the inner portion relative to the outer portion reaches a minimum magnitude indicative of the soundness of the attachment of the two components.
The step of determining whether the displacement reaches a minimum magnitude may advantageously be by measurement of the displacement of the inner portion relative to the outer portion.
Preferably the step of determining whether the displacement reaches a minimum magnitude is by visual inspection of the displacement of the inner portion relative to the outer portion.
Alternatively, the step of determining whether the displacement reaches a minimum magnitude may advantageously be by attempting to engage an indicator element as hereinbefore described with the fastening element, and observing whether the indicator element is retained by the fastening element.
The method may advantageously further comprise the step of determining whether the displacement of the inner portion relative to the outer portion is less than a maximum magnitude, corresponding to a maximum desired magnitude of compressive force.
Preferably the inner portion of the fastening element is tapped and a first end of the tie member has a threaded portion, the step of applying the compressive force being achieved by screwing the tapped inner portion of the element onto the threaded portion of the member.
Preferably the outer portion of the fastening element is formed with flats for engagement with a spanner or like tool, and the step of applying said force is achieved by applying a torque to the fastening element.
The invention also lies in a nut comprising a fastening element as hereinabove described and a bolt for use therewith.